The heart functions to pump blood to the rest of the body. It contains two upper chambers called atria and two lower chambers called ventricles. During each heartbeat, the atria will first contract, followed by the ventricles. The timing of these contractions is important to allow for efficient circulation. This is controlled by the heart's electrical system.
The sinoatrial (SA) node acts as the heart's internal pacemaker and signals the start of each heartbeat. When the SA node fires an impulse, electrical activity spreads through the left and right atria, causing them to contract and squeeze blood into the ventricles. The impulse travels to the atrioventricular (AV) node, which is the only electrical bridge that connects the atrial and ventricular chambers. The electrical impulse propagates through the walls of the ventricles, causing them to contract and pump blood out of the heart. When the SA node is directing the electrical activity of the heart, the rhythm is referred to as normal sinus rhythm (NSR).
Atrial fibrillation (AF) is the most common sustained heart rhythm disorder. Instead of the SA node directing electrical rhythm, many different impulses rapidly fire at once, producing a rapid and highly irregular pattern of impulses reaching the AV node. The AV node acts as a filter to limit the number of impulses that travel to the ventricles, but many impulses still get through in a fast and disorganized manner. As such, the hallmark of AF is an irregular rhythm where the ventricles beat in a very chaotic fashion.
AF presents a major risk factor for stroke. Due to the fast and chaotic nature of electrical activity, the atria cannot squeeze blood effectively into the ventricles. This results in abnormal blood flow and vessel wall damage that increases the likelihood of forming blood clots. If a clot is pumped out of the heart, it can travel to the brain and cause a stroke. People with AF are 5 times more likely to experience a stroke compared to the general population. Three out of four AF-related strokes can be prevented if a diagnosis is available, but many people who have AF don't know it because they may not feel symptoms.
The gold standard for diagnosing AF is the visual inspection of the electrocardiogram (ECG). This relies on AF being present at the time of an in-clinic ECG recording but AF may occur only intermittently. Furthermore, AF would remain undetected in patients who are asymptomatic. As such, there is a need for a more effective screening strategy that can be easily deployed to the general population. The increasing availability of smart phone technology for measuring the blood volume pulse, or plethysmographic waveform presents a growing opportunity for automated detection of AF.
Prior algorithms on automatic AF detection are based primarily on R-R interval variability or rely on the absence of P-waves in the ECG. However, applying these ECG-based techniques to the plethysmographic waveform is non-trivial because the plethysmographic waveform is very different from the ECG. The plethysmogram reflects changes in blood volume as the arterial pulse expands and contracts the microvasculature whereas the ECG reflects electrical activity of the heart. P-waves are not available for analysis in the plethysmogram. Unlike the ECG, the plethysmogram lacks a signature peak such as the QRS complex. This lack of an easily distinguishable peak, coupled with sensor movement, severity of motion artifacts, and the presence of dicrotic notches, pose a significant problem in the accuracy of beat-to-beat interval measurements derived from the plethysmographic waveform. This is particularly problematic when plethysmographic waveforms are acquired remotely in a contact-free manner.
It is therefore desirable to provide a method that can robustly differentiate AF from NSR and other common heart rhythm abnormalities using plethysmographic waveforms. It is further desirable that the method be able to work even with recordings of short duration (under 1 minute).
It is also desirable to have a simple system capable of detecting the possible presence of AF and communicating this condition to the user such that the user is alerted to consult a medical practitioner for further testing and/or treatment.